2.6.2.3.1 Objectives

As stated in Section 2.6.2.1, the primary objective of bioregional assessment (BA) groundwater modelling is to quantify the changes in regional groundwater due to additional coal resource development, which is based on the difference in results between the baseline coal resource development (baseline) and coal resource development pathway (CRDP) simulations. A new regional-scale numerical groundwater model was deemed to be required for this purpose in the Namoi subregion. The main objectives of the regional groundwater model are:

A probabilistic approach of modelling is used which requires the groundwater model to be run thousands of times with different parameter combinations. This can have high computational overheads if the model domain is large and finely resolved. For the Namoi subregion, the modelling domain must encompass all the coal mines and coal seam gas (CSG) developments that are distributed across the subregion (see companion product 2.3 for more details on the coal resource development pathway (Herr et al., 2018)). Given this large domain and the requirement to do thousands of simulations, the groundwater model must be computationally efficient, represent just the key processes for a regional-scale assessment and have a spatial resolution appropriate for representing local- to regional-scale effects of coal resource development.

The model needs to represent the main causal pathway groups that link mine and CSG development hazards to groundwater responses on and off the mine sites (see companion product 2.3 for the Namoi subregion (Herr et al., 2018)):

Key outputs from the model are groundwater drawdowns and changes in surface water – groundwater exchanges, which are summarised as changes in key groundwater and surface water hydrological response variables at model nodes across the modelling domain (Section 2.6.2.1.2.1).

Drawdowns due to additional coal resource development are reported as probability distributions of the differences in drawdown between the CRDP and baseline simulations. The drawdowns are reported by the groundwater model at each model node in the model domain, but since most water-dependent assets access water that is at or near the ground surface in the alluvium and outcropping areas of other geological units, these shallow model nodes that comprise the regional watertable are the model nodes of greatest interest. In addition to the drawdown due to additional coal resource development at the model nodes, probabilistic maps of the drawdown under the baseline and under the CRDP are presented, as well as maps of the difference between the two futures for the regional watertable.

The groundwater model is also used to estimate the change in surface water – groundwater flux to propagate to the surface water models (companion product 2.6.1 for the Namoi subregion (Aryal et al., 2018b). The changes in baseflow from the CRDP and baseline simulations are used in the Namoi river model, where they are incorporated into the hydrological response variables generated as part of the surface water modelling (Aryal et al., 2018b).

Since the main objective of the BA numerical modelling is to quantify the difference between two modelled futures, the emphasis on producing a well-calibrated model is lower than if the objective were to predict the state of groundwater under baseline and under CRDP (Section 2.6.2.1.2). The principle of superposition enables the modelling to focus on the change in hydrogeological stress and the hydraulic properties, rather than on reproducing historical conditions or predicting future state variables of the system, such as groundwater levels or fluxes.

Probabilistic estimates of CSG water production rates and mine water makes are also provided by the groundwater model that are used in comparing the results to the water pumped reported by the proponents’ models (see Section 2.6.2.7). A more comprehensive discussion on the water balance components of the Namoi subregion is presented in companion product 2.5 (Crosbie et al., 2018).